This invention relates generally to electronic modules and more particularly has reference to the use of a self-activated heat pipe for temperature control and automatic mechanical clamping of electronic modules.
The problem of thermal management becomes exceedingly difficult and complex in advanced avionics systems. Such systems typically include a large number of circuit cards mounted in racks or motherboards in a high density fashion, with each card itself containing a high density of electronic components. Power consumptions as high as 150 watts are common. A high efficiency heat removal scheme must be provided to keep the components at a temperature within the range of 20.degree.-100.degree. C. needed for proper operation.
Older avionics systems were air cooled. Arrays of heat conducting fins were provided in the interiors of the circuit card modules. Air coolant was forced across the fins through openings in the modules to extract heat therefrom. That approach had a number of disadvantages which made it undesirable for use in modern advanced avionics systems. A primary disadvantage was excessive power consumption.
Consequently, the industry has now turned to liquid cooled modules. Such systems typically circulate a liquid coolant through cavities formed in the interior of cold plates which support the circuit card racks. Means are provided for communicating heat from the circuit card components to the interface between the rack and the cold plate, which heat is then absorbed and removed by the liquid coolant flowing through the cold plate.
One technique for communicating heat from the components to the interface is to provide a heat pipe with the circuit cards which absorbs heat from the components. A mechanical clamp is used to attach the cold end of the heat pipe to a portion of the rack in thermal communication with the cold plate. See, for example, U.S. Pat. Nos. 4,366,526 entitled, "Heat-pipe Cooled Electronic Circuit Card" and 4,330,812, entitled "Circuit Board Electronic Component Cooling Structure with Composite Spacer". However, the known heat pipe techniques have proved incapable of keeping the circuit components in the desired temperature range. The components become too hot because the heat flow rate is insufficient to handle the large amounts of heat being generated.
It is well known that the thermal resistance at an interface between two bodies is dependent upon the contact pressure along the interface. As contact pressure increases, thermal resistance decreases to allow greater amounts of heat to flow across the interface. One possible explanation for the deficient heat flow rate in the known cooling systems is that the mechanical clamp connecting the heat pipe to the rack creates insufficient pressure at the pipe/rack interface to reduce the thermal resistance therealong. If that is the case, then means for increasing the contact pressure should provide the necessary heat flow rate and should overcome the problems which exists in the known systems.
Thermal switches have been designed to utilize the varying thermal pressure within an integral heat pipe to create a variable contact pressure between the heat pipe and a temperature regulated body. See, for example, U.S. Pat. No. 3,957,107, entitled, "Thermal Switch". However, those devices have been limited to low heat applications because they produce a maximum contact pressure which is far below that needed for efficient heat removal in high heat applications such as advanced avionics systems. Although insufficient contact pressure can be compensated to some degree by highly polished surfaces along the thermal interface, that approach necessitates costly machining steps. Consequently, such devices have not been used in circuit card modules.
A further problem with the known heat pipe systems for cooling circuit card modules is that the heat flow rate remains fixed at the level used for heat removal during normal operation. That level is higher than needed during a warm-up operation. As a result, excessive heat is removed during warm-up and the circuit components are slow to reach normal operating temperatures. A need thus exists for means to provide a variable heat flow rate, i.e., a heat flow rate which is high during normal operation and which is automatically reduced during warm-up operation.
Advanced avionics systems also present problems of maintenance and reliability. Mechanical clamps are currently used to lock the circuit card modules into the rack. Because each module has its own clamps, the maintenance technician for an advanced avionics system must engage in a time consuming process of inspecting and securing great numbers of clamps. Missed clamps can produce system failure or damage. Special installation tools are usually needed to apply the high closure forces associated with the clamps. In addition to increased tooling and maintenance inventory costs, the use of such tools and clamps has frequently caused breakage of modules during routine maintenance and assembly procedures. Hence, it is readily apparent that a need exists for means to lock the modules into the rack without the use of mechanical clamps or installation tools.